(1) Field of the Invention
The present invention relates to an apparatus and method for producing blown tubular films from thermoplastic materials. More particularly, this invention relates to improved tube forming and cooling procedures wherein a blown tube is formed by differential air pressure produced by the action of high velocity cooling air conducted toward and along the external surfaces of the advancing tube as it is being formed.
(2) Description of the Prior Art
Generally, the prior art technique for forming tubular thermoplastic films comprises continuously extruding a melt of a thermoplastic material through an annular orifice, applying internal fluid pressure to the tube thus formed and shape expanding the tube and reducing the wall thickness thereof to appropriate dimensions while cooling and solidifying the extruded thermoplastic. Thereafter, the formed tubing is collapsed by passing it through the nip formed by a pair of counter-rotating pinch rolls. The flattened tubing may be subsequently passed to a wind up station, or on to further processing such as bag-making operations for example.
Although useful tubing has been commercially prepared utilizing this method, under certain circumstances, such a product may have an undesirable gauge non-uniformity, i.e., the thickness of the film is not uniform. Such non-uniform wall thickness results in, for a given average thickness, low gauge points which introduce weak areas in the film. Also, gauge variation results in an uneven, humped roll of film upon winding of the flattened tubing. In addition to the unsightly appearance of such rolls, when the film on such rolls is unwound, it does not lie flat and thus requires special precautions in the printing, conversion and other uses thereof.
One of the major problems in this art is to rapidly cool the extruded bubble of thermoplastic material. Production rate for any given tube (bubble) size is limited by the character of the bubble being extruded. Thus, under a given set of operating conditions, increasing extruder output will cause the thermoplastic to be formed into the tube at a higher rate but since the heat exchange character of the system will not have changed, it will also cause a rise in the height of the frost line (that is the line where the extruded tube turns from molten to solid character). This in turn causes an increase in the instability of the extruded bubble because its unsupported molten length has become too long. Supporting the film bubble in general permits increased cooling air impingement and therefore increased extrusion speeds.